In typical analog video signal source selectors, for example switching matrices or vision switchers, the physical circuit layout is given as much attention as the circuit design of the switching or cross point elements. For example, on printed conductor boards, a ground plane may be interspersed between signal carrying conductors. Similarly grounded conductive layers may be used separate signals likely to contain frequencies having a greater susceptibility to cross coupling to adjacent signal conductors. Often coaxial cable conductors are employed or printed conductors arranged to emulate transmission line characteristics in order to inhibit unwanted coupling of signal energy into adjacent circuits. In a video display device such preventative crosstalk prevention methods may be precluded by virtue of physical space, display layout and product cost. Thus, a video display device which can be capable of receiving multiple baseband analog signal inputs in both standard and high definition TV signal formats from such sources as digital or analog recorders, a satellite TV receiver, DVD player, or computer, may be subject to undesirable and unwanted picture impairment resulting from signal cross coupling.
Component signal formats often comprise a luminance signal with red and blue color difference signals, frequently expressed as Y Pr Pb, and for a digitally derived signal of standard definition, can contain a luminance signal spectra extending to about 5 MHz. Similarly a luminance signal derived from a high definition source can result in a luminance signal spectra extending to 20 MHz and beyond. Typically the color difference signals are constrained to have significantly less bandwidth, typically half or a quarter that of the luminance component and hence these signals pose a less severe crosstalk threat.
Furthermore multiple input signals can occur in both standard and high definition TV signal formats which are asynchronous one with the other, or expressed another way, the plurality of input signals have differing timebase frequencies. Hence, crosstalk susceptible signals will be more obviously displayed as a consequence of the relative motion between the synchronizing signals of the selected signal and the timebase of the unwanted cross coupled component.
In the exemplary switching matrices and vision switchers mentioned previously, input signals are often synchronized or standards-converted to achieve a common timebase frequency. In addition, such synchronized or standards converted signals are usually coupled within the matrix or switcher to enable simultaneous signal selection at multiple destinations. Thus it can be appreciated that the various physical coupling methods described earlier, directed to the prevention of unwanted signal coupling are an essential prerequisite. Hence any residual cross coupling can only exist as a consequence of crosstalk across the signal source selector or switch to produce an unwanted signal at the signal processing destination. Such cross coupling can often occur at the signal source selector by virtue of parasitic capacitance present between open circuit or non-selected switching contacts or elements. Thus signal source selectors frequently have the form of a series shunt combination of switching elements. In this arrangement a series switch element couples or inhibits coupling of the wanted signal to the destination. The shunt switch element is turned on when the series switch element is open thus effectively grounding the output of the series switch and removing any unwanted signals coupled via the parasitic capacitance of the open circuit series switch. A series shunt switching combination is controlled in an inverse manner such that when one switching element is on the other is off, in this way any unwanted crosstalk signals are largely prevented for contaminating the wanted signal source selection.
However, as mentioned previously, the preventative crosstalk techniques employed in video matrices although desirable are generally precluded from use in a video display device by cost and space considerations. As a consequence a video display device capable of selecting between multiple viewing sources, some asynchronous, in both standard and high definition formats can result in severe signal crosstalk with wanted picture degradation. Furthermore, although the series shunt switching arrangement described previously provides a remedy for signal coupling or leakage across the series switch element, any cross coupling occurring prior to the series shunt switching elements is largely unremedied, with the consequence that the selected picture remains impaired.